This invention relates to high pressure sodium vapor and other high intensity discharge lamps comprising an arc tube having disposed therein an inner chamber which is opened on at least one end or has other appropriate venting ports and is operated at relatively high temperatures so as to improve the efficacy and life characteristics of the high pressure discharge lamps.
By way of example, high pressure sodium (HPS) vapor lamps have found wide spread use during the past few decades for commercial lighting applications, especially outdoor lighting. Such lamps are described in U.S. Pat. No. 3,248,590 --Schmidt, High Pressure Sodium Vapor Lamps. HPS vapor lamps typically utilize a slender tubular inner arc tube envelope of a transmissive refractory oxide material resistant to sodium at high temperatures, suitably that of high density polycrystalline alumina or synthetic sapphire. The arc tube typically contains a gas atmosphere comprised of sodium along with other additives such as xenon and mercury.
The luminous efficacy (lumens/watt) of HPS vapor lamps may be improved by increasing the operating temperature of the arc chamber of the arc tube of the HPS lamp. Such increases of and the benefits yielded by operating the arc tube at a temperature of about 1400.degree. C. for a HPS vapor lamp are described in U.S. Pat. No. 4,567,396 of C. I. McVey issued Jan. 28, 1986. While the HPS vapor lamp of U.S. Pat. No. 4,567,396 serves well its intended function, it is desired that further increases in the operating temperature of the arc chamber be accomplished so as to further enhance the efficacy of the HPS lamps without sacrificing life.
Typically increasing the operating temperature beyond a standard practice of about 1200.degree. C. without any special precautions causes the rate of sodium migration out of the arc tube to sharply increase. Such migration reduces the sodium and mercury ratio of this gas within the arc tube, which, in turn, causes the arc voltage within the arc tube to increase. Further, the loss of the sodium reduces the efficacy of the arc tube. The lost sodium, as well as some vaporized aluminum oxide from the arc tube, are . deposited on the vitreous outer envelope of the HPS vapor lamp. Such deposits are manifested as light-absorbing brown layers which decreases the amount of light emitted by the lamp and also increases the amount of radiation retained inside the outer envelope. The retained radiation causes the arc tube temperature to rise, thereby increasing its gas pressure and arc voltage which also results in lowering the efficacy of HPS vapor lamp along with increasing the necessary ballasting conditions to properly operate the lamp. It is desired that the operating temperature of the arc tube be increased beyond the standard design practice of the about 1200.degree. C. limit without suffering the normal resulting serious loss in efficacy through life.
Accordingly, an object of the present invention is to provide an arc tube for a HPS lamp which is capable of being operated at relatively high wall temperature substantially above 1200.degree. C. while at the same time reducing the sodium losses that typically occur for such high temperature operation and the corresponding loss in efficacy.
A further object of the present invention is to provide methods for operating the improved arc tube to achieve enhanced color output of the high pressure sodium vapor lamp.
These and other objects of the present invention will become more apparent upon consideration of the following description of the invention.